Akt (v-akt murine thyoma viral oncogene), also referred to as PKB or Rac, plays a critical role in controlling survival and apoptosis. Franke, T. F. (1997) Cell, 88: 435-437. This serine-threonine protein kinase is activated by insulin and various growth and survival factors, and functions in a wortmannin-sensitive pathway involving PI3 kinase. Burgering, B. T. and Coffer, P. J. (1995) Nature, 376, 599-602; Franke, T. F. et al. (1995) Cell, 81: 727-736. Akt is activated by phospholipid binding and activation loop phosphorylation at Thr308 by PDK1 and by phosphorylation within the carboxy terminus at Ser473. Alessi, D. R. et al. (1996) EMBO J., 15, 6541-6551. The previously elusive PDK2 responsible for phosphorylation of Akt at Ser473 has been identified as mammalian target of rapamycin (mTOR) in a rapamycin-insensitive complex with rictor and Sin1. Sarbassov, D. D. et al., (2005), Science, 307: 1098-1101; Jacinto, E. et al., (2006), Cell, 127: 125-137.
Akt promotes cell survival by inhibiting apoptosis through its ability to phosphorylate and inactivate several targets, including Bad, forkhead transcription factors, c-Raf and caspase-9. Cardone, M. H. et al., (1998), Science, 282: 1318-1321; Brunet, A. et al., (1999), Cell, 96: 857-868; Zimmerman, S. et al., (1999), Science, 286: 1741-1744. PTEN phosphatase is a major negative regulator of the PI3 kinase/Akt signaling pathway. Cantley, L. C. et al., (1999), Proc. Natl. Acad. Sci., USA, 96: 4240-4245. LY294002 is a specific PI3 kinase inhibitor. Vlahos, C. et al., (1994), J. Biol. Chem., 269: 5241-5248. One of the essential functions of Akt is the regulation of glycogen synthesis through phosphorylation and inactivation of GSK-3α and β. Akt may also play a role in insulin stimulation of glucose transport. Hajduch, E. et al., (2000), FEBS Lett., 492: 199-203; Cross, D. A. et al., (1995), Nature, 373: 785-789.).
In addition to its role in survival and glycogen synthesis, Akt is involved in cell cycle regulation by preventing GSK-3β mediated phosphorylation and degradation of cyclin D1, and by negatively regulating the cyclin dependent kinase inhibitors p27 Kip and p21 Wafl. Diehl, J. A. et al., (1998), Genes Dev., 12: 3499-3511; Gesbert, F. et al., (2000), J. Biol. Chem., 275: 39223-39230; Zhou, B. P. et al., (2001), Nat. Cell Biol.; 3: 245-252. Akt also plays a critical role in cell growth by directly phosphorylating mTOR in a rapamycin-sensitive complex containing raptor. Nave, B. T. et al., (1999), Biochem. J., 344: 427-431. More importantly, Akt phosphorylates and inactivates tuberin (TSC2), an inhibitor of mTOR within the mTOR-raptor complex. Manning, B. D. et al., (2003), Biochem. Soc. Trans., 31(3): 573-8. Inhibition of mTOR stops the protein synthesis machinery due to inactivation of its effector p70 S6 kinase and activation of the eukaryotic initiation factor, 4E binding protein 1 (4EEP1), an inhibitor of translation. Manning, B. D. et al., (2002), Mol. Cell, 10: 151-162; Inoki, K. et al., (2002), Nat. Cell Biol., 4: 648-657.
The SH3 domain of the tyrosine kinase Src interacts with a PXXP motif of Akt, and Src activates Akt by phosphorylating Tyr315 and Tyr326. These phosphorylation events are thought to occur prior to the phosphorylation of Thr308 and Ser473. In addition, Src inhibits PTEN, which results in a decrease of dephosphorylation of PtdIns and increased phosphorylation of Akt. Lu, Y. et al., (2003), J Biol Chem., 278(41): 40057-40066; Signoretti et al., J. Nat. Cancer Inst., (2000), Vol. 92(23): 1918.